1. Field of the Invention:
The invention relates to an electro-magnetic and magnetic shielding and to electrical cables provided therewith.
2. Description of the Prior Art: In the field of high frequency applications, it is frequently necessary to enclose one or more signal conductors of an electrical cable within an electro-magnetic shielding. This is to prevent electro-magnetic radiation from the outside from affecting the signal being transmitted, and similarly to prevent radiation of the signal from affecting the outside.
Cables with single-braided shields frequently are not sufficient for high frequency applications, especially in the lower megahertz range. A better shielding effect is achieved by means of two shields that are braided onto each other. In the case of very high requirements for the shielding effect, as necessary for instance in the fields of astronautics, aeronautics, telecommunications and data processing, even the two-braided shielding is not sufficient. For such fields of application, cables having a three-layered shielding have been provided, in which the inner layer and the outer layer are each formed by a metal braiding or conductive foil shield and the intermediate layer is formed by a polycrystalline material, such as for example mu-metal and metallic glass.
However, shielding structures with three layers result in cables having only limited flexibility and difficulties arise in handling these cables when they are connected to contact elements such as plug and socket connectors. In addition, only shielding of electric fields is achieved. Shielding of magnetic fields is not achieved by the three-layer shielding. Moreover, another disadvantage is that the conductive intermediate layer electrically interconnects with the inner layer and the outer layer resulting in the effect of having only one thick-layered shielding.
German-"Offenlegungsschrift" No. 30 25 504 discloses a coaxial cable comprising a magnetic layer between two metal shields, said magnetic layer consisting of a magnetic mixed material which is non-conductive or slightly conductive only and which can be made by mixing ferrite dust or a different magnetic metallic dust into a flexible plastic carrier material.
U.S. Pat. No. 4,376,920 describes two braided shields of a coaxial cable, an intermediate layer with a high dissipation factor in order to achieve a high propagation function for the path between the two shield layers and, thus, a shielding effect that is as length-independent as possible. For the intermediate layer, it is possible to use a plastic material providing good electrical insulation and being loaded with lossy pigments or with other compounds that are not specified in detail.
U.S. Pat. Nos. 3,191,132 and 3,309,633 disclose electrical cables whose flexible insulating plastic material surrounding electrical conductors contains an admixture of ferrite particles in order to obtain an absorption of electro-magnetic waves of high frequency, without having an absorption of such waves in the low-frequency range. For applications of the type mentioned here, the shielding damping should be in the range greater than 100 dB in order to prevent radiation disturbing signals.
Electro-magnetic radiation affecting the signal to be transmitted, have a negative effect, especially in digital signals, in that the pulse edges are flattened. This leads to signal distortions and to a reduction of the possible pulse repetition frequency.
In many electronic fields, pulses of radiation from the cable transmitted to other electronic components o signal conductors of other cables is undesired. In the fields of telecommunications and data processing, undesired cross-talk may occur and unauthorized data tapping is rendered possible due to this radiation.
A problem to be solved by the present invention resides in providing an electro-magnetic shielding having high shielding damping both of electrical and magnetic fields in a frequency range as wide as possible, and in addition providing a highly flexible cable structure.